I would have to agree with Franco about the cost in most cases. There are some projects that this does not hold true for, such as synthetic quilts, but not many.
However, the important part is the making of the gear for me. Long ago MYOG was a way for me to get gear that I could be happy with and proud of. At this point though, I make gear because I enjoy making it. It is a hobby in itself, and one that has actually started to take up more time than my backpacking.